In the recent years, pyroelectric IR sensors have been utilized in a broad range of fields for a variety of purposes such as measurements of temperatures of foodstuffs being cooked in electronic ovens, detection of locations of human bodies in areas cooled with air conditioners. The pyroelectric IR sensors utilize pyroelectric effects obtainable with pyroelectric materials such as LiTaO.sub.3 single crystal.
Pyroelectric material have spontaneous polarization and always produce electric charges on surfaces thereof. However, the pyroelectric materials are kept electrically neutral in their steady states in atmosphere where the electric charges are coupled with those in the atmosphere. When infrared rays are incident on the pyroelectric materials, their temperatures are changed, thereby destroying or changing the neutral state of the electric charges on the surfaces. The pyroelectric IR sensors detect the electric charges produced on the surfaces, thereby measuring quantities of incident infrared rays.
Objects emit infrared rays in quantities which correspond to their temperatures. The pyroelectric effects which result from variations in quantities of incident infrared rays permit measuring temperatures and locations of the objects when infrared rays incident from the objects onto pyroelectric materials are periodically intermitted.
A chopper is used as means for periodically intermitting the infrared rays, and a pyroelectric IR sensor is composed of a pyroelectric member and a chopper. The pyroelectric IR sensor detects temperatures of objects while using the chopper for forcibly intermitting infrared rays incident on the pyroelectric member. An electromagnetic motor or a piezoelectric actuator is mainly used as the chopper.
For obtaining a piezoelectric actuator which is to be used as a chopper, a bonded element is composed by bonding a piezoelectric ceramic member to a thin elastic sheet made of a material such as a metal and one end of the bonded element is fixedly sustained so that it is warped by strain produced by applying a voltage to the piezoelectric ceramic member. A piezoelectric actuator which is composed of a thin elastic sheet having a piezoelectric ceramic members bonded to both surfaces thereof is generally referred to as a bimorph type, whereas a piezoelectric actuator composed of a thin elastic sheet having a piezoelectric ceramic member bonded only to one surface thereof is referred to as a unimorph type. The thin elastic sheet is referred to as an elastic shim member.
A conventional pyroelectric IR sensor which uses a bimorph type piezoelectric actuator as a chopper is configured as shown in FIG. 17.
A reference numeral 10 represents an elastic shim member, reference numerals 11a and 11b designate piezoelectric ceramic members, a reference numeral 12 denotes a shielding plate, a reference numeral 13 represents a stand, a reference numeral 14 designates a fixing member, a reference numeral 15 denotes a pyroelectric sensor unit comprising a pyroelectric member and a reference numeral 16 is infrared rays emitted from an object.
The piezoelectric ceramic members 11a and 11b are bonded to both surfaces of the elastic shim member 10 to compose a bimorph type element. Electrodes are formed on surfaces of the piezoelectric ceramic members 11a and 11b to polarize these members in directions along widths of the members.
The piezoelectric ceramic members 11a and 11b are polarized in directions which are variable dependently on directions of electric fields formed between the elastic shim member 10 and the piezoelectric ceramic sheets 11a and 11b respectively, and directions of polarization are determined so that the piezoelectric ceramic members 11a and 11b produce distortion in directions opposite to each other. Speaking concretely, a direction of an electric field and a direction of polarization are determined so that the piezoelectric ceramic member 11b is distorted to be contracted when the piezoelectric ceramic member 11a is distorted to be elongated.
The bimorph type element is sustained by sandwiching portions of the elastic shim member 10, piezoelectric ceramic members 11a and 11b between the stand 13 and the fixing member 14. The shielding plate 12 is attached to a tip of a free end of the bimorph type element. The pyroelectric sensor unit 15 is disposed in the vicinity of the shielding plate 12 so that it cannot be brought into contact with the shielding plate 12 which oscillates.
When electric fields are applied between the elastic shim member 10 and the piezoelectric ceramic members 11a and 11b respectively, the bimorph type element are warped with one end thereof kept fixed and the shielding plate 12 attached to the tip of the bimorph type element is reciprocally moved as directions of the applied electric fields are changed. The infrared rays 16 incident on the pyroelectric member 15 are intermittently intercepted by the reciprocal motions of the shielding plate 12.